Method of fabricating an organic electroluminescent device

ABSTRACT

The present invention provides a novel organic electroluminescent device having an optical interference member which reduces the overall reflectance from the device. The invention is particularly suited to current-driven organic displays having an anode, an electroluminescent layer and a cathode, where at least one optical interference member is placed between two of the layers and thus forms part of the electrical circuit required to excite the display. The optical interference member is chosen to have a thickness which causes at least some destructive optical interference of ambient light incident on the display. In addition, the material(s) of the optical interference member are chosen to have a work function which is compatible with the highest occupied molecular orbital, or the lowest unoccupied molecular orbital of the electroluminescent layer, depending on the location of the optical interference member in relation to the anode, cathode and electroluminescent layer. The appropriate selection of material can ensure proper current flow the device, thus reducing the likelihood of electrical breakdown of the organic electroluminescent layer, and improving the overall energy efficiency of the device.

RELATED APPLICATION

[0001] The Application of application Ser. No.: 09/361137, filed Jul.27, 1999.

FIELD OF THE INVENTION

[0002] The present invention relates generally electroluminescentdevices and more specifically relates to an organic electroluminescentdevice having a thing film optical interference layer to reducereflectance from ambient light.

BACKGROUND OF THE INVENTION

[0003] Electroluminescent devices (ELDs) are well known and aregenerally constructed of several layers of different materials. Theselayers typically consist of a transparent front-electrode layer, anelectroluminescent layer and a back-electrode layer. When a voltage isapplied across the electrodes, the electroluminescent layer becomesactive, converting some portion of the electrical energy passingtherethrough into light. This light is then emitted out through thefront-electrode, which is transparent to the emitted light, where it isvisible to a user of the device.

[0004] Electroluminescent devices can be particularly useful as computerdisplays and are generally recognized as high-quality displays forcomputers and other electronic devices used in demanding applicationssuch as military, avionics and aerospace where features such as highreliability, low weight, and low power consumption are important.Electroluminescent displays are also gaining recognition for theirqualities in automotive, personal computer and other consumerindustries, as they can offer certain benefits over other displays suchas cathode-ray tubes (“CRT”) and liquid crystal displays (“LCD”).

[0005] One feature of electroluminescent displays is the ability to addthink films between the layers to vary the characteristics of thedisplay. It is known to use thin film layers in electroluminescentdisplays to improve selected display characteristics, such assignal-to-reflected-ambient light ration (“SRA”) and contrast ratio“CR”). For greater clarity, signal-to-reflected ambient light ratio canbe defined as: ${SRA}\quad = \frac{L_{em}}{R \times L_{amb}}$

[0006] where:

[0007] SRA=Signal-to-reflected ambient light ratio

[0008] L_(em)=Emitted luminance of the device

[0009] R=Reflectance of the device

[0010] L_(amb)=The ambient illuminance, or the ambient light incident onthe display

[0011] and, in a pixilated device, contrast ratio can be defined as:${CR} = \frac{L_{on} + {R \times L_{amb}}}{L_{off} + {R \times L_{amb}}}$

[0012] where:

[0013] CR=Contrast Ratio

[0014] L_(on)=Emitted luminance of active or “on” pixels

[0015] L_(off)=Emitted luminance of inactive or “off” pixels

[0016] R=Reflectance of the device

[0017] L_(amb)=The ambient illuminance, or the ambient light incident onthe display

[0018] One particular type of thin-film layer that can be used toimprove contrast ratio in electroluminescent devices is a substantiallytransparent optical interference layer placed between one or more of thelayers of the electroluminescent device, as taught in U.S. Pat. No.5,049,780 to Dobrowolski. As will be apparent to those of skill in theart, improvements to the contrast ratio of an electroluminescent deviceis generally desirable and particularly important in avionics andmilitary applications where poor contrast and glare can have seriousconsequences. Using the principle of destructive interference, theoptical interference layer can result in the reduction of the amplitudeof ambient light by superimposing of two or more, out-of-phase,electromagnetic waves, which can be generated by reflection and/ortransmission at the interfaces of thin-film layer(s). By selectingappropriate thicknesses of the layers, optical destructive interferenceat the electromagnetic wavelengths of interest (typically visibleambient light waves reflected off of the display) can result in anexceptional contrast ratio and/or signal-to-reflected ambient lightratio.

[0019] Dobrowolski is generally directed to voltage-driven inorganicelectroluminescent devices, where the electroluminescent layer is formedof an inorganic material, and which typically require one or moreadditional transparent dielectric layers to reduce electrical-breakdownof the inorganic electroluminescent layer. Such inorganicelectroluminescent devices are typically voltage-driven, powered withalternating current (“ac”) in order to reduce charge build-up within thedevice. While Dobrowolski does generally contemplate the use of directcurrent (“dc”) electroluminescent devices without transparent dielectriclayers, such inorganic devices are still voltage-driven, and aregenerally prone to electrical breakdown of the electroluminescent layer.

[0020] With the advent of modem current-driven organicelectroluminescent devices which offer certain advantages (such as colorimprovements and a reduced barrier to current flow to reduce thenecessary drive voltage) compared to voltage-driven inorganicelectroluminescent devices, there is now a need to improve the contrastratio and/or signal to ambient light ratio of these organic devices, andit can be seen that the prior art does not teach a suitable opticalinterference electroluminescent device to address this need.

SUMMARY OF THE INVENTION

[0021] It is therefore an object of the present invention to provide anovel optical interference organic electroluminescent device whichobviates or mitigates at least one of the disadvantages of the priorart.

[0022] In an embodiment of the invention there is provided an opticalinterference electroluminescent device for displaying an image to aviewer in front of the device, comprising: an anode layer; a cathodelayer, at least one of the anode layer and the cathode layer beingsubstantially transparent to at least a portion of emittedelectroluminescent light; at least one organic electroluminescent layerdisposed between the anode layer and the cathode layer, theelectroluminescent layer having a first energy characteristic being theamount of energy required to extract an electron from a highest occupiedmolecular orbital of the electroluminescent layer, and a second energycharacteristic being the amount of energy required to extract anelectron from a lowest unoccupied molecular orbital of theelectroluminescent layer; and at least one optical interference memberdisposed between two of the layers, and having a work functionsubstantially equal to the first energy characteristic when the opticalinterference member is between the anode and the electroluminescentlayer, and having a work function substantially equal to the secondenergy characteristic when the optical interference member is betweenthe cathode and the electroluminescent layer, the optical interferencemember being of a thickness and material such that the spectralreflectance of the electroluminescent device is so modified that thereflectance of ambient light by the electroluminescent device towardsthe viewer is reduced.

[0023] In another embodiment of the invention, there is provided anelectroluminescent device to emit light in a selected spectrum,comprising: an anode layer, a cathode layer, wherein one of the anodelayer and the cathode layer are substantially transparent to at least aportion of the selected spectrum emitted by the electroluminescentdevice. The electroluminescent device further comprises an organicelectroluminescent layer between the anode layer and the cathode layer,the electroluminescent layer having a highest occupied molecular orbitalrespective to the anode layer and having a lowest unoccupied molecularorbital respective to the cathode layer. The device further includes anoptical interference member having a selected work function and operableto reduce ambient light reflected through the transparent layer, theoptical interference member being between the electroluminescent layerand one of the anode layer and the cathode layer, wherein the differencebetween the selected work function and an energy level required toextract an electron from a respective molecular orbital approaches zero.

[0024] In another embodiment of the invention, there is provided amethod of fabricating an electroluminescent device for displaying animage to a viewer in front of the device, comprising the steps of:

[0025] depositing an anode layer onto a substrate; depositing an organicelectroluminescent layer onto the anode layer, the electroluminescentlayer having a first energy characteristic associated with an anodeside, and a second energy characteristic associated with a cathode side;

[0026] depositing an optical interference member onto theelectroluminescent layer, the optical interference member for reducingthe reflectance of ambient light towards the viewer, the opticalinterference member having a work function substantially equal to thesecond energy characteristic;

[0027] depositing a cathode layer onto the optical interference member;and sealing the device.

[0028] In another embodiment of the invention there is provided a methodof assembling an electroluminescent device for displaying an image to aviewer in front of the device, comprising the steps of:

[0029] depositing an anode layer onto a substrate;

[0030] depositing an optical interference member onto the anode layer;the optical interference member for reducing the reflectance of ambientlight towards the viewer, the optical interference member having a workfunction;

[0031] depositing an organic electroluminescent layer onto the anodelayer, the electroluminescent layer having an energy characteristicbeing the amount of energy required to extract an electron from theelectroluminescent layer, the energy characteristic being substantiallyequal to the work function;

[0032] depositing a cathode layer onto the electroluminescent layer; andsealing the device.

[0033] In another embodiment of the invention, there is provided amethod of displaying an image to a viewer comprising the steps of:

[0034] emitting light from an organic electroluminescent layer betweenan anode and a cathode, said electroluminescent layer having a firstenergy characteristic respective to said anode and a second energycharacteristic respective to said cathode; and

[0035] receiving ambient light incident towards said electroluminescentlayer; and

[0036] forming destructive interference from said ambient light at theincident surface of an optical interference member, said opticalinterference member having a selected work function and disposed betweensaid electroluminescent layer and one of said anode and said cathode,the difference between said work function and a respective energycharacteristic approaching zero.

[0037] The appropriate selection of material of the at least one opticalinterference member ensures proper current flow through the device, thusreducing the likelihood of electrical breakdown of the organicelectroluminescent layer, and improving the overall energy efficiency ofthe device, while still reducing reflectance towards a viewer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present invention will now be described, by way of exampleonly, with reference to certain embodiments shown in the attachedFigures in which:

[0039]FIG. 1 is a schematic diagram of a cross-section of through aportion of an optical interference organic electroluminescent device inaccordance with a first embodiment of the invention; and

[0040]FIG. 2 is a schematic diagram of a cross-section of through aportion of an optical interference organic electroluminescent display inaccordance with a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0041] Referring now to FIG. 1, an electroluminescent device inaccordance with a first embodiment of the invention is indicatedgenerally at 10. Device 10 comprises an electroluminescent transmittinganode 12, an electroluminescent layer 14 disposed behind anode 12, anoptical interference member 16 disposed behind the electroluminescentlayer 14 and a cathode 18 disposed behind the interference member 16.Device 10 is connected to a current source 20 via anode 12 and cathode18 in order to drive a constant current through device 10.

[0042] Electroluminescent transmitting anode 12 is any conductingmaterial which is transparent to at least a portion of emittedelectroluminescent light, such as indium tin oxide (ITO) or zinc oxide(ZnO). In a present embodiment, anode 12 is a layer of indium tin oxidepreferably having a thickness of about fifteen-hundred angstroms (1500Å).

[0043] It is to be understood that electroluminescent transmitting anode12 can have different thicknesses, and can be in the range of, forexample, from about one-thousand angstroms (1000 Å) to aboutthree-thousand angstroms (3000 Å), or from about twelve-hundredangstroms (1200 Å) to about two-thousand angstroms (2000 Å).

[0044] Electroluminescent layer 14 is an organic electroluminescentmaterial such as tris(8-quinolinolato aluminum) (Alq3) or poly(n-vinylcarbozale) (PVC₂) wherein photons of light are emitted when electronsdrop from a lowest unoccupied molecular orbital (“lumo”) of layer 14,where they combine with holes in the highest occupied molecular orbital(“homo”) of layer 14. Accordingly, a current flow throughelectroluminescent layer 14 can produce an emission of light. In apresent embodiment, layer 14 is preferably made fromtris(8-quinolinolato aluminum), preferably having a thickness of aboutone-thousand angstroms (1000 Å), although those of skill in the art willbe able to select other appropriate thicknesses of this layer. As isknown to those familiar with the electrical properties oftris(8-quinolinolato aluminum), the energy required to extract anelectron from the highest occupied molecular orbital is about 5.4electron-volts, which can also be described as the energy required toextract an electron from the surface of layer 14, or, the work functionE_(HOMO) of layer 14. Further, the energy required to extract anelectron from the lowest unoccupied molecular orbital oftris(8-quinolinolato aluminum) is about 3.0 electron-volts, which can bereferred to as E_(LUMO) Thus the difference between E_(HOMO) andE_(LUMO) of tris(8-quinolinolato aluminum) is about 2.4 electron-volts.In other embodiments of the invention, other organic materials can be sochosen (i.e. by selection of material or modification of material withdopants) so that the difference between E_(HOMO) and E_(LUMO) is in therange from about 1.5 electron-volts to about 3.0 electron-volts, whichcovers the spectrum of visible light.

[0045] In a present embodiment, optical interference member 16 comprisesa semi-absorbent layer 16 a and a transparent layer 16 b. Semi-absorbentlayer 16 a is partially reflective, partially absorbing and partiallytransmissive of light in the visible spectrum, and in a presentembodiment, is made from magnesium silver (Mg:Ag) having a thickness ofabout one-hundred-and-eighty-five angstroms (185 Å). Other suitablematerials can include Inconel™, Nickel (Ni), Titanium, or a suitableorganic material and appropriate thicknesses of such layers can bedetermined by those of skill in the art. The extinction coefficient ofthe material and its thickness, should be selected so that thereflection from layer 16 a at a preselected wavelength, neglectingoptical interference, should preferably be at least about thirty-fivepercent, with the remainder of light energy being absorbed anddissipated in the form of heat. Similarly, transmission through layer 16a at a preselected wavelength, neglecting optical interference, willpreferably be at least about thirty-five percent.

[0046] It is to be understood that the extinction coefficient of layer16 a and its thickness can be selected so that the transmission throughlayer 16 a at a preselected wavelength, neglecting optical interference,can be from about thirty percent to about forty percent. Overall, theamount of light transmitted through layer 16 a, after two passes, shouldbe substantially equal to the amount of light that is originallyreflected from layer 16 a, in order to achieve the appropriatedestructive interference at the reflective surface of layer 16 a, aswill be understood by those of skill in the art.

[0047] Substantially transparent conducting lay 16 b is made from indiumtin oxide (ITO) and has a thickness of about eight-hundred-and-fortyangstroms (840 Å). Other suitable materials and layer thicknesses can beused as will occur to those of skill in the art, such as zinc oxide(ZnO). The extinction coefficient of the material of layer 16 b and itsthickness is selected so that the transmission through lay 16 b at apreselected wavelength, neglecting optical interference, is greater thanabout eighty percent, but is preferably at least about ninety percent.As known to those of skill in the art, it is generally preferred thatthe preselected wavelength(s) for layer 16 b should be substantiallyequal to the preselected wavelengths used to choose layer 16 a.

[0048] A wavelength of about five-hundred-and-fifty nanometers (550 nm),the center of the spectrum of visible light, is a presently preferredpreselected wavelength used for the purpose of determining appropriatethicknesses and materials of layer 12 and 14 and member 16, as theresulting device 10 can have the desired optical interferencecharacteristics across the visible light spectrum. As will be understoodby those of skill in the art, an incidental benefit to the selection ofthis wavelength can result in a device which reflects electromagneticenergy outside the visible spectrum, including infra-red, thus reducingthe heating of the display. However, it will occur to those of skill inthe art that other wavelengths can be selected, as desired.

[0049] Optical interference member 16 is also quantifiable in terms ofits work function Φ_(OIM), or the amount of energy required to extractan electron from the surface of member 16. In a present embodiment,positive-charges or holes flow from current source 20 through anode 12and into electroluminescent layer 14, where they combine with electronswhich flow from current source 20 through cathode 18, opticalinterference member 16 and into electroluminescent layer 14. Thus, inorder to facilitate the injection of electrons into the lowestunoccupied molecular orbital of layer 14, the materials chosen formember 16 are such that the difference X between E_(LUMO) and Φ_(OIM),expressed as an absolute value, approaches zero. This expression can beexpressed mathematically as: X = Φ_(OIM) − E_(LUMO)${Where}\quad \begin{matrix}\lim \\{X->0}\end{matrix}$

[0050] In order for the difference X to approach zero, it is believedthat the X can be in the range of from about 0.0 to about 1.5electron-volts (eV). It is believed that the difference X can be in therange of from about 0.1 to about 1.3 electron-volts. More preferably, itis believed that the difference X should be in the range of from about0.6 to about 1.0 electron-volts. In the present embodiment, where layer16 a is made from magnesium silver and lay 16 b is made from indium tinoxide, the overall work function Φ_(OIM) of member 16 is about 3.6electron-volts. As E_(LUMO) is about 3.0 electron-volts, the differenceX is about 0.6 electron-volts.

[0051] Cathode 18 is magnesium silver having a thickness of aboutfive-hundred angstroms (500 Å), and in the present embodiment isreflective. In other embodiments, it is believed that cathode 18 canhave a thickness between about two-hundred-and-fifty angstroms (250 Å)to about two-thousand-angstroms (2000 Å). However it will occur to thoseof skill in the art that other suitable conducting materials andthicknesses can be used.

[0052] It now will be apparent that the thickness and material(s) ofmember 16 and its components can be determined with an operation thatconsiders, at least in part, the thicknesses of and/or materials ofanode 12, electroluminescent layer 14, and cathode 18.

[0053] Device 10 can be fabricated using techniques known in the art. Inthe foregoing embodiment, anode 12 is vacuum-deposited onto a glasssubstrate, and the subsequent layers are formed thereon also usingvacuum deposition. The entire device 10 is then sealed using techniquesknown in the art. Other suitable substrates and means of fabricatingdevice 10 will occur to those of skill in the art. For example, thesubstrate can be plastic. Further, where electroluminescent layer 14 ispoly(n-vinyl carbozale), then spin-coating can be an appropriatefabrication technique for layer 14.

[0054] The operation of device 10 will now be discussed. It will beappreciated by those of skill in the art that the following is asimplified model for purposes of explanation, and that other physicalphenomena occurring during the operation of device 10 are assumed, forthe purposes of this discussion, to have a negligible influence on theoperation. Current source 20 is ‘on’, so that holes are driven intoelectroluminescent layer 14 via anode 12. These holes then combine withelectrons delivered into layer 14 from source 20, via cathode 18 andoptical interference member 16. The fact that the difference betweenE_(LUMO) and Φ_(OIM) expressed as an absolute value, approaches zero,reduces barriers to current flow through device 10, preventing orinhibiting breakdown of layer 14. The recombination of holes andelectrons in layer 14 causes light to be emitted out through the frontor exterior face of anode 12 and towards a viewer, as indicated by arrowL_(em).

[0055] At the same time, ambient light is incident upon device 10, asindicated by arrow L_(amb) and passes through anode 12 andelectroluminescent layer 14. Ambient light L_(amb) incident uponsemi-absorbing layer 16 a is partially reflected, partially absorbed andpartially transmitted. The light transmitted through semi-absorbinglayer 16 a passes through transparent layer 16 b, where it reflects offcathode 18 and back through transparent layer 16 b, at which point thisreflected light is inverted one-hundred-and-eighty degrees out of phasewith the partially reflected light from layer 16 a, and thus these tworeflections destructively interfere and substantially cancel each otherout. The energy otherwise found in these two reflections is absorbed bysemi-absorbing layer 16 a and cathode 18, where it is dissipated as arelatively small amount of heat. The result is that reflected light(L_(ref)) back towards the viewer from device 10 is reduced. In apresent embodiment, reflected light (L_(ref)) is reduced by about ninetypercent, compared to an electroluminescent device assembled withoutoptical interference member 16.

[0056] It is believed that in other embodiments of the invention,reflected light (L_(ref)) can be reduced by as much as about 99.5percent by choosing different materials, thicknesses and extinctioncoefficients for optical interference member 16 and by selectedappropriate thicknesses and materials for the other layers in device 10,although still within the aforementioned acceptable parameters andranges such that the difference between E_(LUMO) and Φ_(OIM), expressedas an absolute value, approaches zero. However, while a higherdifference in energy levels between E_(LUMO) and Φ_(OIM), still withinthe acceptable ranges, can achieve reduced reflection, it can alsoresult in reduced electrical efficiency in the current flow throughdevice 10.

[0057] In other embodiments of the invention, a suitably modifiedoptical interference member 16 can be disposed in series with otherlayers of device 10. Referring now to FIG. 2, an electroluminescentdevice in accordance with a second embodiment of the invention isindicated generally at 10 a. Like components to those shown in FIG. 1are indicated with like reference numbers. Device 10 a comprises aelectroluminescent transmitting anode 12, an optical interference member16′ disposed behind anode 12, an electroluminescent layer 14 disposedbehind interference member 16′, and a cathode 18 disposed behindelectroluminescent layer 14. Device 10 a is connected to a currentsource 20 via anode 12 and cathode 18 in order to drive a constantcurrent through device 10 and layer 14.

[0058] Electroluminescent transmitting anode 12 is any conductingmaterial which is transparent to at least a portion of emittedelectroluminescent light, such as indium tin oxide (ITO) or zinc oxide(ZnO). In a present embodiment, anode 12 is a layer of indium tin oxidepreferably having a thickness of about fifteen-hundred angstroms (1500Å).

[0059] It is to be understood that electroluminescent transmitting anode12 can have different thicknesses, and can be in the range of, forexample, from about one-thousand angstroms (1000 Å) to aboutthree-thousand angstroms (3000 Å), or from about twelve-hundredangstroms (1200 Å) to about two-thousand angstroms (2000 Å).

[0060] Electroluminescent layer 14 is an organic electroluminescentmaterial such as tris(8-quinolinolato aluminum) (Alq3) or poly(n-vinylcarbozale) (PVC_(z)) wherein photons of light are emitted when electronsdrop from a lowest unoccupied molecular orbital (“lumo”) of layer 14,where they combine with holes in the highest occupied molecular orbital(“homo”) of layer 14. Accordingly, a current flow throughelectroluminescent layer 14 can produce an emission of light. In apresent embodiment, layer 14 is preferably made fromtris(8-quinolinolato aluminum), preferably having a thickness of aboutone-thousand angstroms (1000 Å), although those of skill in the art willbe able to select other appropriate thicknesses of this layer. As isknown to those familiar with the electrical properties oftris(8-quinolinolato aluminum), the energy required to extract anelectron from the highest occupied molecular orbital is about 5.4electron-volts, which can also be described as the energy required toextract an electron from the surface of layer 14, or, the work functionE_(HOMO) of layer 14. Further, the energy required to extract anelectron from the lowest unoccupied molecular orbital oftris(8-quinolinolato aluminum) is about 3.0 electron-volts, which can bereferred to as E_(LUMO). Thus the difference between E_(HOMO) andE_(LUMO) of tris(8-quinolinolato aluminum) is about 2.4 electron-volts.In other embodiments of the invention, other organic materials can be sochosen (i.e. by selection of material or modification of material withdopants) so that the difference between E_(HOMO) and E_(LUMO) is in therange from about 1.5 electron-volts to about 3.0 electron-volts, whichcovers the spectrum of visible light.

[0061] In a present embodiment optical interference member 16′ comprisesa substantially transparent conducting layer 16 c made from indium tinoxide (ITO) and having a thickness of about seven-hundred-and-forty-fiveangstroms (745 Å). Other suitable materials can be used as will occur tothose of skill in the art, such as zinc oxide (ZnO) or a suitableorganic material. The indium tin oxide of layer 16 c is modified (bycontrolling its stoichiometry) so that it behaves as aquarter-wave-stack at a light wavelength of five-hundred-and-fiftynanometers (550 nm), and chosen so that the transmission through layer16 c is greater than about eighty percent, and preferably at least aboutninety percent. However, it will occur to those of skill in the art thatother wavelengths can be selected, as desired.

[0062] As discussed previously, optical interference member 16′ is alsomeasurable in terms of work function Φ_(OLM). In a present embodiment,holes flow from current source 20 through anode 12 and opticalinterference member 16′, and into electroluminescent layer 14, wherethey combine with electrons which flow from current source 20 throughcathode 18 and into electroluminescent layer 14. Thus, in order tofacilitate the extraction of electrons from the highest occupiedmolecular orbital of electroluminescent layer 14, the materials chosenfor member 16′ are such that the difference Y between E_(HOMO) andΦ_(OIM), expressed as an absolute value, approaches zero. Thisexpression can be expressed mathematically as: Y = Φ_(OIM) − E_(HOMO)${Where}\quad \begin{matrix}\lim \\{Y->0}\end{matrix}$

[0063] In order for the difference between E_(HOMO) and Φ_(OIM) toapproach zero, it is believed that the difference Y can be in the rangeof from about 0.0 to about 1.5 electron-volts (eV). It is believed thatthe difference Y can be in the range of from about 0.1 to about 1.3electron-volts. More preferably, it is believed that the difference Yshould be in the range of from about 0.4 to about 1.0 electron-volts. Inthe present embodiment, where layer 16 a is made from magnesium silverand layer 16 b is made from indium tin oxide, the overall work functionΦ_(OIM) of member 16 is about 5.0 electron-volts. Since E_(HOMO) isabout 5.4 electron-volts, the difference Y is about 0.4 electron-volts(eV).

[0064] Cathode 18 is magnesium silver having a thickness of aboutfive-hundred angstroms (500 Å), and in the present embodiment isreflective. In other embodiments, it is believed that cathode 18 canhave a thickness between about two-hundred-and-fifty angstroms (250 Å)to about two-thousand-angstroms (2000 Å). However, it will occur tothose of skill in the art that other suitable conducting materials andthicknesses can be used.

[0065] It now will be apparent that the thickness and material(s) ofmember 16′ and its components can be determined with an operation thatconsiders, at least in part, the thicknesses of and/or materials ofanode 12, electroluminescent layer 14, and cathode 18.

[0066] Device 10 a can be fabricated using techniques known in the art.In the foregoing embodiment, anode 12 is vacuum-deposited onto a glasssubstrate, and the subsequent layers are formed thereon also usingvacuum deposition. The entire device 10 a is then sealed usingtechniques known in the art. Other suitable substrates and means offabricating device 10 a will occur to those of skill in the art. Forexample, the substrate can be plastic. Further, where electroluminescentlayer 14 is poly(n-vinyl carbozale), then spin-coating can be anappropriate fabrication technique for layer 14.

[0067] The operation of device 10 a will now be discussed. It will beappreciated by those of skill in the art that the following is asimplified model for purposes of explanation, and that other physicalphenomena occurring during the operation of device 10 a are assumed tohave a negligible influence on the operation. Current source 20 is ‘on’,so that holes or positive-charges are driven into electroluminescentlayer 14 via anode 12 and optical interference member 16. Thesepositive-charges then combine with electrons delivered into layer 14from source 20 via cathode 18. Because the difference between E_(HOMO)and Φ_(OIM) approaches zero, barriers to current flow through device 10a are reduced, preventing or inhibiting breakdown of layer 14. Therecombination of electrons and holes in layer 14 causes light to beemitted out through the front face of anode 12 and towards a viewer, asindicated by arrow L_(em).

[0068] At the same time, ambient light is incident upon device 10 a asindicated by arrow L_(amb) and passes through anode 12 and is incidentupon transparent layer 16 c. About half of ambient light L_(amb)incident upon layer 16 c is reflected, while the remainder istransmitted. The light transmitted through layer 16 c reflects off thesurface of electroluminescent layer 14 and back through layer 16 c. Dueto the quarter-wavelength thickness of layer 16 c, this reflected lightfrom layer 14 is inverted one-hundred-and-eighty degrees out of phasewith the reflected light from layer 16 c, and thus these two reflectionsdestructively interfere and substantially cancel each other out. Theenergy otherwise found in these two reflections is transmitted throughlayer 16 c. The result is that reflected light (L_(ref)) back towardsthe viewer from device 10 is reduced. In the present embodiment,reflected light (L_(ref)) is believed to be reduced by a range of fromabout 0.5 percent to about two percent, compared to electroluminescentdevices without layer 16′.

[0069] While only specific combinations of the various features andcomponents of the present invention have been discussed herein, it willbe apparent to those of skill in the art that desired sub-sets of thedisclosed features and components and/or alternative combinations ofthese features and components can be utilized, as desired. For example,the embodiments discussed herein can be combined to provide multipleoptical interference members disposed between different layers of theelectroluminescent device, and therefore disjoined from each other, inorder to further reduce reflectance from the device. For example, layer16′ and layer 16 can be included on respective sides of layer 14.

[0070] Furthermore, each optical interference member can be atransparent layer or a combination of a transparent layer and asemi-absorbing layer in order to achieve different results, and it willbe apparent that these different types of optical interference memberscan also be placed at different locations throughout the device. Forexample, it is contemplated that a transparent layer, without asemi-absorbing layer can be used between the organic electroluminescentlayer and the cathode, and similarly, a combination of a transparentlayer and a semi-absorbing layer can be used between the anode and theelectroluminescent layer, by having the difference between work functionof the optical interference member and the energy required to extract anelectron from either highest occupied molecular orbital, and/or thelowest unoccupied molecular orbital, approach zero, as appropriate.

[0071] The present invention can be suitable for a computer display. Forexample, a pixilated organic electroluminescent computer display can beformed where the anode comprises a plurality of generally parallel andspaced anodes to compose the front layer of an organicelectroluminescent computer display, and the cathode comprises a numberof spaced cathodes which are generally perpendicular to the anodes. Itwill be further understood that the anode and the cathode can bepatterned in a variety of ways, other than pixilated, to createdifferent recognizable patterns to a user of device 10. When such adisplay is pixilated or patterned, it will be appreciated thatindividual pixels or patterns can be fired using known techniques suchas pulsed-DC, and/or adding a periodic reverse-polarity ‘refresh’ pulseto reduce built-up charge. The device can also be hybrid-display havingan active matrix, as can be found in notebook computers.

[0072] In addition, the present invention can be suitably modified foruse in color organic electroluminescent devices. As known to those ofskill in the art, such multi-color and full-color devices can be formedfrom stacked transparent organic electroluminescent layers. Where theoptical interference member is between two of these stacked layers, itwill be apparent that the difference between its work function and theenergy level required to extract an electron from the lowest unoccupiedmolecular orbital of the electroluminescent layer nearest the anodeshould approach zero, and the difference between the work function andthe energy level required to extract an electron from the highestoccupied molecular orbital of the electroluminescent layer nearest thecathode should also approach zero. More preferably, the work function ofthe optical interference member should be the average of the lowestunoccupied molecular orbitals of the surrounding electroluminescentlayers.

[0073] Multi-colored and full-colored devices can also be providedthrough a patterned red-green-blue organic layer (i.e. by selectingmaterials having inherent color properties, or by appropriately dopingthe patterns on the layer). Other colorizing techniques can includingthe use of a white-emitter and appropriate filters. It will be apparentthat the teachings of the present invention can be modified toaccommodate these and other color devices.

[0074] The present invention can be suitable for use as a backlight fora liquid crystal display.

[0075] It will also be understood that the present invention can besuitably modified for organic electroluminescent devices which have anelectron transport layer between the cathode and the electroluminescentlayer, and/or a hole transport layer between the anode and theelectroluminescent layer. In this embodiment, the optical interferencemember can thus be placed between the transport layer and theelectroluminescent layer, or the transport layer can be placed betweenthe electroluminescent layer and the optical interference member. Inthis embodiment of the invention, it will be apparent that the opticalinterference member is still selected to have a work function such thatthe difference between the work function and the energy levelsassociated with the respective anode side and/or cathode side of theelectroluminescent layer(s) approach zero, in order to facilitatecurrent flow through the entire device. It is also contemplated that anappropriate transport layer can be incorporated into an opticalinterference member.

[0076] Furthermore, while the embodiments herein refer to a front anodeand a rear cathode, it will be apparent that the present invention canbe suitable to a device having a front cathode and a rear anode, as longat least the front layer is transparent to at least a portion of emittedelectroluminescent light.

[0077] The present invention provides a novel organic electroluminescentdevice having an optical interference member which reduces the overallreflectance from the device. The optical interference member is selectedto have a thickness which causes at least some destructive opticalinterference of ambient light incident on the display. In addition, thematerial(s) of the optical interference member are chosen to have a workfunction such that the difference between the work function and theenergy level required to extract an electron from the highest occupiedmolecular orbital, or the lowest unoccupied molecular orbital of theelectroluminescent layer approaches zero, depending on the direction ofcurrent flow and location of the optical interference member in relationto the electroluminescent layer. The appropriate selection of materialfor the optical interference member improves proper current flow throughthe device, thus reducing the likelihood of electrical breakdown of theorganic electroluminescent layer and improving the overall energyefficiency of the device. Finally, in embodiments where a semi-absorbentlayer and transparent layer are combined to form the opticalinterference member, then placement of such an optical interferencemember in contact with the electrode at the back of the device canactually increase the reflectance of infra-red ambient signatures,compared to absorbing films, thus reducing the heating of the displayand reducing the likelihood of damage to the electroluminescent layer.

29. A method of fabricating an electroluminescent device for displayingan image to a viewer in front of said device, comprising the steps of:depositing an anode layer onto a substrate; depositing an organicelectroluminescent layer onto said anode layer, said electroluminescentlayer having a first energy characteristic associated with an anode4side, and a second energy characteristic associated with a cathode side:depositing an optical interference member onto said electroluminescentlayer, said optical interference member for reducing the reflectance ofambient light towards said viewer, said optical interference memberhaving a work function substantially equal to said second energycharacteristic; depositing a cathode layer onto said opticalinterference member; and sealing said device.
 30. A method of assemblingan electroluminescent device for displaying an image to a viewer infront of said device, comprising the steps of: depositing an anode layeronto a substrate; depositing an optical interference member onto saidanode4 layer; said optical interference member for reducing thereflectance of ambient light towards said viewer, said opticalinterference member having a work function; depositing an organicelectroluminescent layer onto said anode layer, said electroluminescentlayer having an energy characteristics being the amount of energyrequired to extract an electron from said electroluminescent layer, saidenergy characteristic being substantially equal to said work function;depositing a cathode layer onto said electroluminescent layer; andsealing said device.